For a stent-graft implanted inside of a blood vessel such as an artery, a certain degree of shape-maintainability, also referred to as ‘rigidity’ or ‘stiffness,’ is required in order to avoid contingent transformations of the stent-graft, such as excessive dents or bending, owing to a movement of the blood vessel or a change of blood flow/blood pressure after the stent-graft has been implanted in the blood vessel. At the same time, a degree of ‘flexibility’ is also required in order for a stent-graft to be able to tightly attach to an inner wall of the blood vessel, and to follow a possible movement from expansion or shrinkage of the blood vessel.
Stent-grafts that place emphasis on shape-maintainability are known as ‘stiff’ stent-grafts. One example of a ‘stiff’ stent-graft is the so-called ‘z stent’ that is comprised of an elastic metal stent surrounding a tubular graft in a triangular wave form, as disclosed in patent document 1. A second example of a ‘stiff’ stent-graft is a stent-graft having a rhombic mesh structure, as disclosed in patent document 2.
However, when using a ‘stiff’ stent-graft, it can be difficult to get the stent-graft to flexibly follow the movement of the blood vessel, or to follow the form of a curve-shaped blood vessel. These shortcomings can lead to poor adhesive properties between the stent-graft and the blood vessel that may result in blood entering gaps between the stent-graft and the inner wall of the blood vessel. In addition, in the case of the ‘z type’ stent-graft there is a chance for a pointed part to form in a bent stent that might break or otherwise damage the blood vessel.
On the other hand, a stent-graft disclosed in the patent document 3 is known as a ‘flexible type’ stent-graft that places emphasis on ‘flexibility.’ This stent-graft has an arrangement that is referred to as a ‘helical type,’ wherein a tubular graft is supported by a stent comprising multiple elastic rings arranged at predetermined intervals.
The ‘flexible type’ stent-graft has high adhesive properties to the blood vessel; however, due to movement of the blood vessel or due to changes of the blood flow/blood pressure, there exists a tendency with flexible stent-grafts for the opening of the stent-graft to tilt and become diagonally inclined, as opposed to perpendicular, in relation to the axial direction of the blood vessel (hereafter referred to as being diagonally inclined). In the event that an opening of a stent-graft becomes diagonally inclined, a gap is formed between the stent-graft and the inner wall of the blood vessel. Consequently, blood might enter the gap, or the stent-graft might move as a result of being pushed out of place by the blood entering the gap. Furthermore, in the event that the opening of a stent-graft is diagonally inclined, an interval between some rings located at the vicinity of the opening (end-part) on one side surface of the rings may become shorter than an interval between the same rings on the opposite side surface.
In the event that the above-mentioned problems occur, it is frequently necessary to perform corrective treatment on an already implanted stent-graft, or to conduct a complete replacement of a faulty stent-graft.
In light of the above, an idea of a ‘hybrid type’ stent-graft that combines both ‘flexibility’ and ‘stiffness’ was created and is disclosed in patent document 4 and in patent document 5. These ‘hybrid type’ stent-grafts have an arrangement wherein ‘z-type’ arrangements are used only for the two opposite end-parts of the stent-graft, while a ‘helical-type’ arrangement is used in a middle part of the stent-graft.
A practical problem that arises with the use of ‘hybrid type’ stent-grafts, as the ones described in patent documents 4 and 5, is that due to their structure, it is questionable whether or not these stent-grafts can in fact be transported with conventional unobtrusive methods to an affected target area. This problem is described in detail below.
Folding a ‘hybrid-type’ stent-graft, such as the ones described in patent documents 4 and 5, in a radial direction for the purpose of housing it in a delivery sheath can pose a challenge. This is because when the ‘z-type’ end-parts, and ‘helical-type’ middle parts are made to shrink in the radial direction, they fold in a substantially different manner, which leads to conflicting forces that resist one another along the body of the hybrid stent-graft, and particularly at the sections of the stent graft where the ‘z-type’ parts and ‘helical-type’ part meet.
The ‘z-type’ parts of the stent-graft have a considerably high rigidity or shape maintainability, therefore folding the ‘z-type’ parts in the radial direction is done by forcing toward each other the zig-zagging wires, whose walls appear as mountains and valleys, to be substantially parallel to each other. Thus when the ‘z-type’ parts of the stent are folded they exert a pulling force in the radial direction.
On the other hand, folding the ‘helical type’ part of the ‘hybrid type’ stent-graft in the radial direction involves folding the ‘helical type’ part of the stent-graft in what is referred to as a saddle-shape. The saddle-shape inevitably forces the helical part of the stent-graft to transform in both an axial direction as well as a radial direction. This movement in the axial direction conflicts with the rigid structure of the ‘z-type’ parts of the stent-graft.
Therefore, in the case of folding the ‘hybrid-type’ stent-graft, as described in the patent documents 4 and 5, in order to shrink the stent-graft in the radial direction, a discrepancy occurs as follows. The axial movement of the ‘helical type’ part of the hybrid stent-graft conflicts with the axial movement from the ‘z-type’ parts of the ‘hybrid-type’ stent-graft. As a result, with consideration of the conflicting forces described above, it can be expected that folding the ‘hybrid-type’ stent-grafts described in documents 4 and 5 will be extremely difficult in reality. Some degree of folding and shrinking of the stent-graft in the radial direction is likely possible with the exertion of force, but even under severe pressure, the diameter of the folded hybrid stent-graft would likely be too large to be housed in a delivery sheath for delivering the stent-graft to an affected target area.
i. In support of the above, it is worth noting that the documents 4 and 5 do not disclose any detailed view of their respective hybrid stent-grafts being housed in a delivery sheath.